In order to able to fly a rotorcraft, the pilot makes use of a plurality of on-board instruments, and in particular of a vertical speed indicator. This indicator determines the instantaneous vertical speed (vertical speed upwards or downwards) of the rotorcraft and informs the pilot.
The vertical speed indicator relies on the principle of measuring variation in atmospheric pressure while climbing or descending. By way of example, this can be done by an indicator constituted by a pressure gauge capsule connected to a thermally insulated tank, the assembly being put into communication with the surrounding air (ambient atmosphere) via a calibrated capillary tube.
The variable speed indicator is controlled by the pressure difference between the surrounding air (atmospheric pressure or static pressure within the indicator but outside the capsule) and the pressure inside the capsule via the connection provided by the capillary tube acting as an air intake of determined bore size. The capillary tube needs to be calibrated so that the deformation of the capsule represents the pressure difference between the local pressure at a given instant and the local pressure at the immediately preceding instant. Consequently, the capillary tube needs to be sufficiently fine to ensure that while climbing or descending the static pressure (ambient atmosphere) differs sufficiently from the pressure inside the capsule.
In other words, the capillary tube imposes a certain delay in atmospheric pressure becoming established throughout the capsule-and-tank assembly.
Given the principle on which it operates, it can readily be understood that the indications given by a vertical speed indicator are not instantaneous whenever there is an upward or downward change in the flightpath of an aircraft in flight.
Since the vertical speed indicator responds only to pressure variations, it naturally results during level flight that the pressures inside and outside the indicator equalize via the capillary tube so that the indicated rate of climb or descent becomes equal to zero.
In addition, it is essential for the pilot to be aware of speed VA relative to air, generally referred to as “airspeed”. This is measured and displayed by an airspeed indicator. That instrument is a differential pressure gauge that measures the difference between the static pressure and the total pressure of the air flow past corresponding pressure intakes.
The static pressure (ambient pressure at the static pressure intake) is independent of the airspeed of the aircraft.
The total pressure (or “stagnation pressure”) is obtained from an intake for receiving the total pressure of the air flow.
Under such conditions, the airspeed indicator also includes an aneroid capsule that deforms to a greater or lesser extent as a function of the magnitude of the difference between the total pressure and the static pressure.
In general, the static and total pressure intakes are grouped together on a single probe known as a Pitot tube. A Pitot tube is substantially streamlined and cylindrical, with a leading end that is generally hemispherical. The Pitot tube is placed on the aircraft so that firstly the total pressure intake is situated at the extreme leading end of the cylindrical body, and secondly the static pressure intake is radial and located behind the total pressure intake.
In the special case of a rotorcraft, the static pressure intake is in principle located along the fuselage whereas the total pressure intake is situated at the leading end of a pole of greater or lesser length.
Thus, and in application of Bernoulli's theorem, which theorem is valid in particular at the forward speeds of a rotorcraft, said difference is equal to a dynamic pressure (proportional to the square of the airspeed of the aircraft) from which the indicated airspeed of the aircraft is deduced.
This speed is delivered an on-board instrument, i.e. an indicator for constituting an airspeed installation so that the indicated speed VA corresponds:                on the ground, to the speed of the aircraft relative to the ambient atmosphere; and        at altitude (in flight), to the equivalent airspeed, i.e. the product of the true airspeed VV (or flightpath speed) multiplied by the square root of the relative density σ of the air, itself equal to the quotient of the density ρ of air at the altitude in question divided by the density ρ0 of air at ground level in a “standard atmosphere”, i.e.:        
      V    A    =                    V        V            ⁢              σ              =                  V        V            ⁢                        ρ                      ρ            0                              
In reality, the indicated airspeed differs from the equivalent airspeed because of instrument error. Consequently, the indicated airspeed needs to be corrected by calibrating the airspeed installation so as to correspond to the corrected speed or calibrated airspeed VC that is close to but different from the equivalent airspeed.
Such an instrument is calibrated solely for “standard atmosphere” conditions at sea level. In other words, the indicated speed is, in fact, equal to the speed relative to the air only if the pressure is 101,325 pascals and if the temperature is 15 degrees Celsius (ρ=ρ0).
When the actual atmosphere is significantly different from the standard atmosphere, a correction is introduced based on “density altitude” that need not be described herein.
It is important to observe that traditional devices with a Pitot tube and a static pressure intake present sensitivity that tends to zero when the airspeed of the aircraft decreases.
Furthermore, for reasons of clarity, if a proper airspeed VP is defined as being equal to the horizontal component of the true airspeed, i.e. VV·cos θ where θ is the slope angle of the flightpath of the aircraft. Thus, the vector {right arrow over (VP)} and the vector {right arrow over (VW)}, i.e. the horizontal component of the wind, produce a geometrical resultant corresponding to the ground speed vector {right arrow over (VS)} that is fundamental for navigation purposes. Naturally, the true airspeed VV and the proper airspeed VP are equal in level flight. Ignoring instrument errors, it is thus possible to assume in level flight that the calibrated airspeed VC is equal to the product of the proper speed VP multiplied by √{square root over (σ)}, i.e.:VC=VP√{square root over (σ)}
Furthermore, with rotorcraft, first and second flying speed regimes are defined. A flying speed at the boundary between those two speed regimes is referred to as the “minimum-power” speed and is equal to about 65 knots (kt), for example. It corresponds to the lowest level of power needed for level flight, which level is referred to as the “minimum” power. This is a minimum presented by a curve plotting the power required for a rotorcraft to fly in level flight as a function of its forward speed. This necessary power is the sum of:                the induced power associated with the lift that needs to be produced, and equal to the product of the so-called “induced” speed multiplied by the dynamic lift: this power level decreases with increasing forward speed of the rotorcraft;        the profile power due to the profile drag of the blades of the main rotor, which power varies with varying blade profile: this power level increases with increasing forward speed;        the fuselage power due to the drag of the fuselage: this power level increases rapidly as a function of speed, substantially as a function of speed raised to the third power; and        power losses due in particular to the transmission of power from the engine to the main and tail rotors, to cooling, to driving accessories: these power losses increase with increasing forward speed of the rotorcraft.        
Thus, the first speed regime applies when the calibrated airspeed VC of the rotorcraft is greater than the minimum-power speed VY. In this regime the power required increases with increasing airspeed and corresponds to flight that is stabilized.
In contrast, the second speed regime applies below said minimum-power speed. In this regime rotorcraft flight suffers from instability. In this second speed regime, the calibrated airspeed is low and the power required increases as the speed of the rotorcraft decreases. Airspeed indicator measurements then become less and less reliable as the forward speed of the rotorcraft decreases. In addition, the measured instantaneous vertical speed is approximate, as explained above, because of the delay associated with the inertia of a vertical speed indicator.
Furthermore, a phenomenon known as “uplift” can falsify interpretation of the indications given by a vertical speed indicator.
Usually, the uplift phenomenon is a natural phenomenon involving air moving towards a higher altitude.
Thus, when the pilot causes the nose of the rotorcraft to rise, even only a little, and possibly instinctively, but without changing the instantaneous engine power (energy) of the rotorcraft, the vertical speed indicator begins by indicating a vertical speed that is positive.
In the short term, the longitudinal attitude of the fuselage increases and the rotorcraft tends to climb. However the total energy of the rotorcraft is the sum of its kinetic energy plus its potential energy. Since power is being maintained constant, increasing potential energy results in decreasing kinetic energy decreases, so the rotorcraft slows down.
Unfortunately, the pilot is unaware of the loss of speed of the rotorcraft, believing that advantage is being taken of the natural phenomenon of uplift. The pilot therefore does not remedy this loss of speed by increasing the power of the rotorcraft.
Furthermore, as explained above, such slowing down within the second regime of speeds needs to be accompanied by an increase in the amount of power.
As a result, vertical speed drops off suddenly and becomes highly negative, since the pilot has not increased the power being delivered, as is required because of the decrease in forward speed. The rotorcraft thus begins to drop quickly and possibly dangerously in a manner that the pilot does not foresee since only a few instants previously the vertical speed indicator was indicating a positive vertical speed.
In addition, when flying without visibility, the pilot needs to rely completely on the information provided by the instruments available in the instrument panel, including the vertical speed indication. If an emergency situation arises, the pilot may be caused to respond hurriedly. Such a hasty reaction can furthermore be accentuated by the said information becoming available rather late. The pilot may then commit piloting errors that can lead to an accident. This can happen if the pilot, worried by the presence of a nearby obstacle, acts involuntarily by reflex on the cyclic pitch stick without also increasing engine power. This leads to the rotorcraft taking on a slightly nose-up attitude followed by a rapid loss of altitude that is extremely dangerous if the rotorcraft is flying close to the ground or to water.
Furthermore, it should be observed that document EP 0 006 773 describes a method for determining a predictive speed for an aerodyne.